1. Field of the Invention
This invention relates generally to a method and system is functioning properly. In particular, the present invention relates to optical fire detection systems wherein ultraviolet light detector tubes are used as sensors to detect the presence of fire.
2. Description of the Related Art
Fires produce radiation that can be detected by ultraviolet light detector tubes. Ultraviolet light detector tubes are often used as sensors in optical fire detection systems for aircraft and other vehicles. Typically, a high voltage potential is applied to the ultraviolet light detector tube. When ultraviolet radiation hits the detector tube, the radiation generates current pulses at the cathode of the detector tube. The rate of the current pulses increases as the intensity of the radiation increases. Circuits can be constructed to measure the radiation detected and determine if the radiation has reached a level equal to the level of ultraviolet radiation present with a fire.
Fire detection systems using ultraviolet light detector tubes generally require that the detector tubes be tested to insure the system is fully functional. However, in most applications detector tubes cannot be tested by simple electrical means because they exhibit nearly infinite resistance when they are not detecting ultraviolet light. When ultraviolet light is not present, a high voltage potential (1100 volts or more) must be placed across the detector tube to test it. One method known in the art for testing detector tubes is the use of an ultraviolet light emitter. An ultraviolet light emitter is a lamp that produces ultraviolet radiation when activated. To test whether the detector tube operates properly, an ultraviolet light emission lamp is permanently placed next to the detector tube. During a self test the emission lamp is activated to emit ultraviolet radiation that should trigger an alarm signal if the system is working properly If the fire detection system does not respond with a signal indicating a fire, then either the detector tube or the detector electronics are defective.
While emitter lamps provide a method to test whether a detector tube functions properly, there are several disadvantages with using emitter lamps. First, emitter lamps add to the cost and size of fire detection systems. The cost and size are increased by having to add the emitter lamps themselves. Each detector tube in the system requires an emitter lamp located adjacent to the detector tube for testing purposes The detector tubes are commonly placed in several locations. Placing emitter lamps next to each ultraviolet light detector tube doubles the wiring requirements. Second, the activation of the emitter lamps requires additional power. In addition to the high voltage potential required for the detector tubes, more power is required for the emitter lamps. Third, emitter lamps tend to have reliability problems. If an emitter lamp is unreliable it puts the operational status of the entire fire detection system in question. This is a significant disadvantage since ultraviolet light emitters are more unreliable than detector tubes. A system test may indicate a defective detector tube when in fact only an emitter lamps is defective. Therefore, the use of emitter lamps adds to the cost, weight and unreliability of optical fire detection systems.